Pneumatic rubber tires conventionally have rubber sidewalls composed of carbon black reinforced rubber compositions which are, therefore, black in color.
Sometimes it is desired to provide a decorative white colored rubber composition on a portion of an outer surface of such tire sidewall. Such rubber composition is conventionally colored white with titanium dioxide pigment and is devoid of, or does not contain, carbon black.
Sidewalls of rubber tires may age somewhat prematurely due to (i) weathering due to atmospheric conditions such as, for example, exposure to ultraviolet light, ozone and high humidity, (ii) fatigue cracking due to continual flexing of the tire sidewall under operating conditions and (iii) abrasion due to scuffing against road curbs or other objects.
White colored tire sidewalls are of special consideration because, (ii) due to the white color of the sidewalls, the weathering, fatigue cracking and abrasion may be more cosmetically observable than for black colored sidewalls and (ii) due to the absence of carbon black reinforcement the white rubber composition (a) before curing has a lower viscosity and, therefore, is a softer composition to process in rubber mixing and extrusion equipment and, also, has a greater tendency to flow which can be a problem for a manufactured article, prior to curing and (b) after curing typically has less abrasion resistance to scuffing.
In practice, for white sidewalls, weathering is typically retarded by (i) use of antidegradants and, sometimes, the use of low unsaturation rubbers such as, for example, halobutyl rubber--particularly chlorobutyl rubber, and EPDM's, in the rubber composition in the white sidewall rubber composition. EPDM rubbers are well known to those skilled in such art and, generally, mean ethylene/propylene terpolymer elastomers, with a minor amount of non-conjugated diene (e.g. 2 to 15 percent of the terpolymer).
Natural cis 1,4-cis polyisoprene rubber is sometimes used in white sidewall rubber compositions in order to enhance green strength of the uncured rubber composition as well as its building tack and to aid in the processing of the uncured rubber composition during a relatively low shear mixing process typically experienced in mixing the white sidewall rubber composition.
For some white sidewall applications, it is desired to use synthetic cis 1,4-polyisoprene rubber instead of natural cis 1,4-polyisoprene rubber in the white sidewall rubber composition which is, as hereinbefore described, devoid of carbon black reinforcement. While motivation for such substitution may be for a multiple of reasons ranging from (i) concern about visible impurities in natural rubber such as, for example, minor amounts of fiber and dirt impurities from the rubber tree plantation source of the natural rubber to (ii) occasional increased cost considerations for natural rubber as compared to synthetic natural rubber. The fact that the white sidewall rubber is devoid of carbon black reinforcement may also be a motivation because, as hereinbefore discussed, without the carbon black reinforcement, the white sidewall composition is relatively soft and, therefore, processes differently in mixing and extrusion and typically has less green strength and, therefore, has a greater tendency to flow prior to curing.
However, synthetic natural rubber normally provides less toughness, abrasion resistance and fatigue-to-failure time, than natural rubber for such a white rubber sidewall composition.
Accordingly, it is an aspect of this invention to enable a satisfactory substitution of synthetic cis 1,4-polyisoprene rubber for at least a portion of natural cis 1,4-polyisoprene in a white sidewall rubber composition.
As will be hereinafter described, such substitution is accomplished by use of a combination of synthetic cis 1,4-polyisoprene rubber and trans 1,4-polybutadiene rubber.
It should be pointed out that it has relatively recently been proposed to use trans 1,4-polybutadiene rubber in tire sidewall compositions, particularly in carbon black reinforced rubber compositions, in order to increase resistance to flex fatigue and tear initiation and/or increase resistance to cut growth.
Considerations of such suggestions of use of trans 1,4-polybutadiene in tire sidewall composition may be found, for example, in U.S. Pat. No. 5,386,865.
However, it is not known to the inventor where trans 1,4-polybutadiene rubber is used in a white sidewall rubber composition devoid of carbon black reinforcement where it is desired that the rubber composition contains natural rubber but where a combination of trans 1,4-polybutadiene polymer and synthetic cis 1,4-polyisoprene is used to an exclusion, or substantial exclusion, of natural cis 1,4-polyisoprene rubber.